1. Field of the Invention
The present invention relates to a multilayer ceramic substrate used for a high-density multilayer circuit substrate.
2. Description of the Related Art
A multilayer ceramic substrate produced by firing at low temperatures (hereinafter, referred to as a multilayer ceramic substrate) is used for a high-density circuit substrate. The multilayer ceramic substrate is produced by stacking a plurality of ceramic green sheets, each of which has a conductive pattern formed by a conductor composition, and firing the laminate of the ceramic green sheets. To connect the conductive pattern of one layer to that of the other layer, through holes (via holes) are formed in the green sheet of the desired layer beforehand, and then conductor paste (i.e., the conductor composition) is filled into the via holes. Thereafter, the laminate that includes the green sheet having the through holes filled with the conductor paste is fired. In this manner, the green sheets and the conductor paste are fired simultaneously so as to form an electrode in the via holes. Consequently, the conductive patterns of the desired layers are connected, thereby providing a three-dimensional circuit.
However, in a conventional firing, there are many cases where the firing shrinkage behavior of a conductor differs from that of ceramics. This makes it difficult to produce a large flat substrate. Moreover, a variation in shrinkage during firing also makes it difficult to produce a substrate with high dimensional accuracy.
To solve the above problems, the following method is proposed by JP 2785544 B2. Another green sheet is prepared by using an inorganic composition that is not sintered at the firing temperature of each of the green sheets constituting a laminate. This green sheet thus prepared is stacked on the upper and the lower surface of the laminate as constraint layers, and then fired. According to this method, the shrinkage of the green sheet laminate in the in-plane direction is restricted by the constraint layers, so that the shrinkage selectively occurs only in the thickness direction. Thus, a substrate with favorable flatness and dimensional accuracy can be achieved. The constraint layers are not sintered even after firing and can be removed easily.
However, the above method using the constraint layers is likely to cause defects at the interface between a conductor and ceramics due to a difference in sintering timing or firing shrinkage behavior between the conductor and the ceramics in a firing process. Specifically, the defects are caused easily between an internal electrode and a ceramic body and between a via electrode and the ceramic body. These defects significantly reduce the reliability of the substrate.
A method that does not use the constraint layer can suppress the above defects because the shrinkage occurs three-dimensionally in a firing process. Even if such defects are caused, they are small enough to be repaired fully during firing. However, when the constraint layer is used, the shrinkage in the in-plane direction hardly occurs. Therefore, once defects are caused, it is almost impossible to repair the defects, so that they remain even in the final stage.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a multilayer ceramic substrate that can achieve sufficient flatness and high dimensional accuracy, while preventing defects that occur in the vicinity of electrodes after firing.
A multilayer ceramic substrate of the present invention includes a glass ceramic body, a conductive pattern, and a via conductor. The conductive pattern is formed in the glass ceramic body and on at least one principal surface of the glass ceramic body. The via conductor makes a connection between two or more of the conductive patterns. The via conductor includes a conductive material and a Mo compound or a Mo metal. The conductive material includes at least one selected from the group consisting of Ag, Au, Pt and Pd as a main component. The amount of Mo compound or Mo metal is in the range of 0.05 to 10 parts by weight in terms of Mo metal with respect to 100 parts by weight of the conductive material. This multilayer ceramic substrate can achieve sufficient flatness and high dimensional accuracy, while preventing defects in the vicinity of electrodes.
It is preferable that oxide particles that include at least one selected from the group consisting of alumina, zirconia and magnesia as a main component are arranged on the at least one principal surface of the glass ceramic body. This configuration can prevent solder leaching.
A multilayer ceramic substrate according to another example of the present invention is produced by: forming a laminate by stacking a plurality of green sheets so that at least one of the green sheets has via holes filled with a conductor composition; stacking a constraint green sheet on both surfaces of the laminate, the constraint green sheet being made of ceramics that is sinterable at a temperature higher than a firing temperature of the laminate; and firing the laminate with the constraint green sheets. The green sheets may be made of glass ceramics. The conductor composition includes conductive powder and a Mo compound or a Mo metal. The conductive powder may include at least one selected from the group consisting of Ag, Au, Pt and Pd. The amount of Mo compound or Mo metal is in the range of 0.05 to 10 parts by weight in terms of Mo metal with respect to 100 parts by weight of the conductive powder. This multilayer ceramic substrate can achieve sufficient flatness for mounting semiconductors, chips, etc. and high dimensional accuracy, while preventing defects in the vicinity of electrodes. Therefore, the electrical characteristics of the multilayer ceramic substrate are not degraded, making it highly reliable.
It is preferable that the conductor composition further includes a glass frit in an amount of not more than 10 parts by weight. The addition of the glass frit can increase the adhesive strength in firing the conductor composition and adjust the final contraction of the conductor composition to be produced.
The conductor composition further may include a frit of an inorganic composition used in the glass ceramics in an amount of not more than 10 parts by weight.
The amount of Mo compound or Mo metal included in the conductor composition may be not less than 0.05 parts by weight and less than 3 parts by weight in terms of Mo metal.
It is preferable that the glass frit has a softening point of not less than 650xc2x0 C. This can eliminate the deviation in sintering timing between the green sheet and the conductor composition and prevent defects in the vicinity of electrodes.
A method for manufacturing a multilayer ceramic substrate of the present invention includes the following: forming a laminate by stacking a plurality of green sheets so that at least one of the green sheets has via holes filled with a conductor composition; stacking a constraint green sheet on both surfaces of the laminate, the constraint green sheet being made of ceramics that is sinterable at a temperature higher than a firing temperature of the laminate; and firing the laminate with the constraint green sheets. The green sheets may be made of glass ceramics. The conductor composition includes conductive powder and a Mo compound or a Mo metal. The conductive powder may include at least one selected from the group consisting of Ag, Au, Pt and Pd. The amount of Mo compound or Mo metal is in the range of 0.05 to 10 parts by weight in terms of Mo metal with respect to 100 parts by weight of the conductive powder. This method can provide a multilayer ceramic substrate that can achieve sufficient flatness for mounting semiconductors, chips, etc. and high dimensional accuracy, while preventing defects in the vicinity of electrodes. Therefore, the electrical characteristics of the multilayer ceramic substrate are not degraded, making it highly reliable.
It is preferable that the constraint green sheet is removed after firing the laminate so that a portion of the constraint green sheet is left as particles. This method can provide a multilayer ceramic substrate that can prevent solder leaching.
A multilayer ceramic substrate of the present invention is produced by using a green sheet and a conductor composition. The green sheet is made of glass ceramics. The conductor composition includes conductive powder and a Mo compound or a Mo metal. The conductive powder includes at least one selected from the group consisting of Ag, Au, Pt and Pd. The amount of Mo compound or Mo metal is in the range of 0.05 to 10 parts by weight in terms of Mo metal with respect to 100 parts by weight of the conductive powder. This multilayer ceramic substrate can achieve sufficient flatness for mounting semiconductors, chips, etc. and high dimensional accuracy, while preventing defects in the vicinity of electrodes. Therefore, the electrical characteristics of the multilayer ceramic substrate are not degraded, making it highly reliable.